Perhaps that's it! My 581 seems to do that as well. It definitely operates differently than the 4 year old 560 I have (which still works very well, too). Perhaps that's why I've gotten up to 2-1/2 hours. run time from my 3300 mAh battery, on approx. 850 sq. ft., comprised of hardwood and 2 areas rugs.
On rare occasions, the 581 will only clean 1/2 of the dining room and prematurely move into the next room. However, on its return trip to HB, it will clean the 1/2 of the dining room that it missed. It definitely concentrates on the 1/2 that it missed, so it seems its mapping something!
The 560, with supposedly the same battery, only gets about 1 hr. 15 mins. doing the same area. I say "supposedly the same battery" because I purchased it from a different source, and it is not actually marked as 3300 mAh on the battery, so I'm not sure if it actually is, or if I was sent a lesser battery. The battery I purchased from Metapo has been great, so far it's 8 mos., and really no discernable difference in run time.
The robot did it...